A. Field of the Invention
The invention relates to the field of plant genetics and, more specifically, to the development of carrot plants having increased lycopene content.
B. Description of Related Art
Carrot (Daucus carota var sativus), a member of the Umbelliferae family, is one of the most important root crops and is grown on over 100,000 hectares worldwide. Carrots provide an excellent source of various vitamins and minerals, including vitamin A (beta-carotene), as well as dietary fiber content in animal diets. Breeding efforts over the last half century have resulted in a 75% increase in the beta-carotene content in cultivated carrots. Recently, interest in the nutritional value of other carrot pigments, such as xanthophylls, lycopene, anthocyanins, and other phytochemicals, has increased. Despite recent breeding efforts, there remains a need for the development of carrots having increased levels of carotenoids, other than beta-carotene, such as lycopene.
In general, the goal of vegetable breeding is to combine various desirable traits in a single variety/hybrid. Such desirable traits may include greater yield, resistance to insects or pests, tolerance to heat and drought, better agronomic quality, higher nutritional value, growth rate and fruit properties.
Breeding techniques take advantage of a plant's method of pollination. There are two general methods of pollination: a plant self-pollinates if pollen from one flower is transferred to the same or another flower of the same plant or plant variety. A plant cross-pollinates if pollen comes to it from a flower of a different plant variety.
Plants that have been self-pollinated and selected for type over many generations become homozygous at almost all gene loci and produce a uniform population of true breeding progeny, a homozygous plant. A cross between two such homozygous plants of different varieties produces a uniform population of hybrid plants that are heterozygous for many gene loci. Conversely, a cross of two plants each heterozygous at a number of loci produces a population of hybrid plants that differ genetically and are not uniform. The resulting non-uniformity makes performance unpredictable.
The development of uniform varieties often involves the development of homozygous inbred plants, the crossing of these plants, and the evaluation of the crosses. Pedigree breeding and recurrent selection are examples of breeding methods that have been used to develop inbred plants from breeding populations. Those breeding methods combine the genetic backgrounds from two or more plants or various other broad-based sources into breeding pools from which new lines are developed by selfing and selection of desired phenotypes. The new lines are evaluated to determine which of those have commercial potential.
Carrots (Daucus carota) are one of the most important root crops and is grown on over 100,000 hectares worldwide. There are two main types of cultivated carrots. Eastern/Asiatic carrots are often called anthocyanin carrots because of their purple roots, although some have yellow roots. They typically have pubescent leaves giving them a gray-green color and bolt easily. They have slightly dissected leaves with branched roots and are an annual plant. Western or carotene carrots typically have orange, red or white roots. These carrots were most likely derived from the first group by selection among hybrid progenies of yellow Eastern carrots, white carrots and wild subspecies grown in the Mediterranean. The leaves are generally strongly dissected with unbranched roots and bright green, sparsely hairy foliage and are biennial.
The biennial carrot is a plant that only flowers every two years. In the first year the plant produces the edible root and a leafy top. If a carrot plant is left in the ground for another year, aided by a resting and cold vernalization period, it flowers and seeds are produced. Sexual reproduction in carrots can therefore be carried out as with other flowering plants.
The Western carrot is the most popular carrot and is sub-divided into three groups: 1) short-rooted varieties that mature more quickly; such as Amsterdam Forcing, Tiana, Early French Frame, Early Nantes, Champion Scarlet Horn; 2) medium-rooted varieties, which are the most common type of commercially grown carrots and include varieties such as Mokum, Flakkee, Autumn King, Chantenay Red Cored, Royal Chantenay; and 3) long-rooted varieties, such as New Red Intermediate and Saint Valery.
Carrots are widely used as a fresh market or processed product. As a crop, carrots are grown commercially wherever environmental conditions permit the production of an economically viable yield. Carrots are highly regarded for their nutritional value and their storability. Carrots provide an excellent source of various vitamins and minerals, including vitamin A (beta-carotene), as well as dietary fiber content. Recently, interest in the nutritional value of other carrot pigments, such as xanthophylls, lycopene, anthocyanins, and other phytochemicals, has increased.
Historically, most carrot breeding methods involved mass selection and pedigree selection resulting in a great number of open-pollinated carrot varieties (Stein and Nothnagel, 1995). The first carrot hybrids were sold in the 1960s in the United States following the detection and analysis of male sterility in carrot by Thompson (1961) and Hanschke and Gabelman (1963). Hybrid breeding in carrot is generally based on two systems of cytoplasmic male sterility (CMS) with different genetic backgrounds and origin: “brown anther” type and “petaloid” type (Stein and Nothnagel, 1995). A third CMS system has been detected in an alloplasmic form originating from a cross between the wild carrot D. carota gummifer Hook. fil. and the cultivated carrot D. c. sativus Hoffm. (Nothnagel, 1992).
While breeding efforts to date have provided a number of useful carrot lines with beneficial traits, there remains a great need in the art for new lines with further improved traits. Such plants would benefit farmers and consumers alike by improving crop yields and/or quality.